APU systems are typically used as adjuncts to primary engines, in aircrafts and the like. An APU, typically in the form of an independent secondary gas turbine, provides shaft power to drive interconnected equipment requiring constant speed operation. Often, an APU is used to drive an electrical generator forming part of an APU system. APUs and APU systems are often used as secondary power sources when an associated primary engine is not fully operational. For example APU systems may be used to provide back-up power to the electrical power system of an aircraft, while grounded or during an in-flight emergency.
Often APU systems include gearboxes used to match the speed and torque requirements of interconnected generators. Most typically, the gearbox hard-couples the APU to the generator. During transient conditions, such as, for example, when the APU system is brought on and off-line, the gearbox and APU may be subjected to high impact torques.
Accordingly, the use of one or more clutches to disconnect the gearbox has been suggested. Such a clutch arrangement is, for example, disclosed in U.S. Pat. No. 5,174,109.
The APU generator, however, is often used to provide power to an operational electrical system. Mere use of a clutch will not necessarily guard against mechanical shocks imparted as a result of electrical phase and frequency differences between the APU generator output and the operational electrical system.
Accordingly, an improved APU system allowing electrical generators to be brought on-line and off-line, smoothly without imparting or being the subject of unnecessary torques is desirable.